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Social jet lag calculator: how far apart are your workday and free-day sleep?
For people whose weekend nights start and end much later than their weeknights and who want to see the size of that shift. Enter both sleep times to get the gap between your workday and free-day mid-sleep, its band and your body clock's low point.
Workdays 11:00 PM–7:00 AM, free days 1:00 AM–10:00 AM
Why this number?
Social jet lag calculator
Workday nights
Free-day nights (no alarm)
Use the time you expect to fall asleep, not when you get into bed. Nights that cross midnight are fine.
Can you catch up on sleep on weekends? What the numbers say →
There’s no perfect night, only a better-planned one.
The Lull app has the sleep calculator from our home page, plus 6 sleep sounds that stop a minute before your phone’s alarm.
See the Lull appSay you sleep from 11:00 PM to 7:00 AM on workdays and 1:00 AM to 10:00 AM on free days. The middle of your sleep moves 2 h 30 min later on free days, which puts you in the calculator's top band.
What the gap does not measure
The gap describes timing, not health. It cannot know how much sleep you need, how long you take to fall asleep or why your week looks as it does, and the bands are our own way to read the size, not published cut-offs. People who work shifts, look after a baby or have a sleep condition do not fit a simple workday and free-day split, so the number means little for them. The body-clock low point is a typical figure from the CDC Yellow Book, not a measurement of you.
See a doctor if you feel tired all day whatever you do, if your sleep is broken most nights, or if you regularly take more than 30 minutes to fall asleep (NIOSH lists that among the symptoms to raise with a health care provider). This page is general information, not medical advice: read our disclaimer.
How the gap is measured
Mid-sleep is the halfway point between falling asleep and waking. The calculator finds it for a workday night and for a free-day night and gives the difference between them. That is the measure Roenneberg and colleagues used in their large study of social jet lag (2012): the difference between mid-sleep on free days and on workdays. The times go round a 24-hour clock, so a night that crosses midnight is handled, and the answer says whether free-day sleep sits later or earlier.
The size is then read against three bands, which are our own rule of thumb: under 1 h, 1 h to 2 h, and over 2 h. No official body gives cut-offs for social jet lag. From 6 h the calculator asks you to check the times, because a gap that large usually means a mistyped time or a schedule that is not a simple workday and free-day split.
Worked example: late weekends
On workdays you fall asleep at 11:00 PM and wake at 7:00 AM: 8 h of sleep, with mid-sleep at 3:00 AM. On free days you fall asleep at 1:00 AM and wake at 10:00 AM: 9 h, with mid-sleep at 5:30 AM. The gap is 2 h 30 min, later on free days, in the top band. Had you woken at 7:00 AM on free days too, keeping the 1:00 AM bedtime, the gap would be 1 h. A smaller example: workdays 11:30 PM to 7:00 AM and free days 12:30 AM to 9:00 AM give 1 h 30 min, the middle band.
What the gap and its band mean
The large figure is the gap. The boxes give both mid-sleep times, how much longer or shorter you sleep on free days, and the band. Under them, each line links the method, the bands and the body-clock window to their rows on the methodology page.
The values behind this gap
Each row is one value this tool uses, with its tier: Primary = read off the source page, Secondary = reported by the source from another body, Studio rule = our own choice, with the reason given on the methodology page. The links in the result above go to the same rows there.
| Value | Tier | Source |
|---|---|---|
| Bands for the gap between free-day and workday mid-sleep low under 1 h, middle 1 h to 2 h, high over 2 h; check the times from 6 h | Studio rule | Our rule, leaning on Roenneberg et al., Current Biology, 2012 |
| Body clock low point (circadian nadir) 2 to 4 h before your usual wake time | Primary | CDC Yellow Book, 2025 |
| Time to fall asleep that NIOSH lists as a symptom to raise with a health care provider 30 min | Primary | CDC/NIOSH, 2020 |
Quotes, notes and the date we last read each source are on the methodology and sources page.
Your body clock's low point
Social jet lag is a gap in the timing of your body clock. The CDC Yellow Book puts the clock's daily low point, the circadian nadir, about 2 to 4 hours before your usual wake time, and says bright light after it tends to move the clock earlier while light in the evening tends to move it later. The result shows that window for your free-day wake time. In the worked example, free-day sleep ends at 10:00 AM, so the low point falls around 6:00 AM to 8:00 AM. It is a typical figure for a person, not a measurement of yours.
What research says
Wittmann and colleagues (2006, Chronobiology International) named the mismatch between the timing the body clock prefers and the timing a work or school week imposes “social jetlag”. In their questionnaire study, late sleepers showed the biggest differences between work and free days. Roenneberg and colleagues (2012, Current Biology) measured it as the difference in mid-sleep, as this calculator does, in a large population study, and found it associated with a higher body mass index. Both are observational studies: they show patterns across many people, not what a smaller gap would do for any one person, and this page makes no claim that it would.
Social jet lag is not sleep debt
The two are easy to mix up because both compare workdays and free days. Social jet lag is about when you sleep: how far the middle of your sleep moves. Sleep debt is about how much: the hours you fall short of your need. Someone can sleep a full night every night and still have a large gap, if their free-day nights simply start and end later. To count hours short of your need, use the sleep debt calculator. For crossing real time zones, the jet lag planner gives night-by-night times.
Social jet lag questions
What is social jet lag?
The gap between when your body clock would have you sleep and when your work or school week makes you sleep. Wittmann and colleagues named it in 2006. It is measured, as here, as the difference between your mid-sleep on free days and on workdays.
How is social jet lag calculated?
Find the halfway point of your sleep on a workday and on a free day, then take the difference. Workdays 11:00 PM to 7:00 AM and free days 1:00 AM to 10:00 AM give mid-sleep at 3:00 AM and 5:30 AM: a gap of 2 h 30 min.
How much social jet lag is a lot?
There is no official cut-off. The calculator uses our own bands: under 1 h, 1 h to 2 h, and over 2 h. They describe the size of the gap, nothing more.
What is the body clock low point in the result?
The CDC Yellow Book puts the circadian low point about 2 to 4 hours before your usual wake time. The result shows that window for your free-day wake time.
Is social jet lag the same as sleep debt?
No. Social jet lag is a clock-time gap between workday and free-day sleep. Sleep debt is the hours you fall short of your need, which the sleep debt calculator adds up.
Are my sleep times stored anywhere?
No. The gap is worked out in your browser. The share link holds only the four times, and only if you copy it.
More questions about the site, its sources and privacy are answered in the Lull FAQ.